FIG. 1 is a diagram illustrating a circuit of an active rectifier using a conventional active diode.
An active rectifier 10 substitutes the conventional bridge diode circuit by using the metal-oxide-semiconductor field-effect transistor (MOSFET), and can reduce a conduction loss by using a MOSFET, thereby being integrated by using IC technology. The active rectifier 10 composed of an active diode 20 is composed of M1, which is a switch of the MOSFET, and due to the characteristics of the MOSFET, a parasitic diode D1 is located between drain and source as illustrated in FIG. 1. A comparator 21 turns on M1 if VKA, which is the voltage between cathode (K) and anode (A) (VKA), is lower than 0 V. Since the comparator 21 cannot provide, for itself, the sufficient current to operate the M1 at a high speed, the comparator 21 operates the M1 by using a gate driver 22 that has the sufficient capability to provide a current.
FIG. 2 is a waveform diagram illustrating an operation when the active diode in FIG. 1 operates.
If a VKA becomes lower than 0, an output of a comparator 21 becomes high, and M1 is turned-on through a gate driver 22. Here, due to the delay time td1+td2 of both the comparator 21 and the gate driver 22, after the VKA becomes lower than zero and td1+td2 passes, the M1 is turned-on. Thus, since a current flows through D1 during the period of td1+td2, the efficiency of the active diode 20 is slightly reduced. Also, electromagnetic interference (EMI) may occur, which is caused by the sudden change of the VKA as the M1 is turned-on.
FIG. 3 is a diagram illustrating an active diode performing a turn-on delay compensation using offset; FIG. 4 is a diagram illustrating an operation waveform in the case the delay compensation using offset is performed; and FIG. 5 is a diagram illustrating a phenomenon that a hard turn-on of M1 is performed when a VKA is greater than 0 and lower than Voff.
FIG. 3 is a case of a comparator 21 being operated by applying an offset. Because the output of the comparator 21 becomes high due to Voff before VKA becomes zero, the point in time when M1 is turned-on is brought to an earlier time after td1+td2 as illustrated in FIG. 4, so as to acquire more ideal characteristics of a diode. However, as in FIG. 5, even when the VKA is greater than 0 and lower than Voff and such a condition is maintained, there may be the phenomenon that the M1 is turned-on so as to reversely turn on an active diode 20. Here, a MOSFET is fully turned-on, thus Rdson being very low, which is the resistance between drain and source, so that a lot of currents may flow. Such a phenomenon may cause a loss that is not made by a normal operation of a diode but by an abnormal operation thereof.